In many applications, it is desirable or even necessary to estimate the location of an object with a high accuracy. There are many systems that are designed to allow a device to carry out location estimation using radio frequency (RF) signals. E.g., a device can estimate its location using signal strength of received RF signals, such as the method described in U.S. Pat. No. 7,515,578. The angle of arrival of the received signal can also be used for determining the location of the receiver.
The time of flight (TOF), also known as time of arrival (TOA), can also be used for location estimation. In the example shown in FIG. 1, the TOA based location estimation is typically carried out using trilateration, i.e., the location of an object is estimated based on the distances between the object to be estimated and some objects with their position known. Typically, because the target device is not synchronized to the anchor devices, the time of flight needs to be estimated using a round-trip flight time. For example, a first device transmits a first ranging signal first; then, after receiving the first ranging signal, the second device transmits a second ranging signal. The round trip delay is estimated by the first device. Such a technique is often referred to as Two-Way TOA (TW-TOA) and is commonly used in many systems. Unfortunately, TW-TOA requires a large number of transmissions among all nodes and as a results, a TW-TOA based system cannot accommodate many mobile devices. The large number of devices also results in higher power consumption of the nodes.
Alternatively location estimate can be performed using time difference of flight (TDoF), also known as time difference of arrival (TDOA). In an example shown in FIG. 2, a mobile node 102 broadcasts a radio signal and the signal is received by the receivers of all anchor nodes 101 within its range. If we denote the arrival time of the signal to anchor node i and j as ti and tj respectively, the time difference of the arrival time δti,j=tj−ti is recorded, instead of the absolute time ti and tj.
The TDOA described above has a significant advantage over TOA, because it only requires the mobile node to transmit once and the anchor nodes only need to receive.
In the system illustrated in FIG. 2, because only the target devices are required to transmit, the system has better efficiency and can admit a larger number of target devices in a single coverage area. However, in prior systems, the target devices were not synchronized and the transmissions may collide. Moreover the anchor nodes need to be synchronized. This synchronization is accomplished by adjusting all of the anchor nodes to a common reference timing source. Typically, a synchronization unit is used to generate the timing reference signal and distributes it to all anchor nodes via cables. A major drawback of such a system is the complexity and subsequently installation cost. It also suffers performance degradation as the density of target devices increases.